The term “threaded connections” means any assembly constituted by substantially tubular elements, which are metallic and capable of being coupled together by makeup, in particular with a view to constituting either a stem for drilling a hydrocarbon well or a work-over riser or string for operating such wells, such as a riser, or a casing or tubing string used in operating a well.
Each tubular component comprises an end portion provided with a male threaded zone or a female threaded zone for making up with a corresponding end portion of an analogous element. When coupled, the elements make up what is known as a connection.
These threaded tubular components of a connection are coupled under predetermined loads in order to satisfy clamping and sealing requirements imposed by the service conditions. Further, it should be understood that the threaded tubular components may have to undergo several makeup-breakout cycles, in particular in service.
The conditions for use of such threaded tubular components give rise to a wide range of mechanical loads which act on the sensitive portions of these components, such as the threaded zones, abutting zones or sealing surfaces over wide ranges.
For this reason, connection design is a complex specification which demands taking several parameters into account at the same time. Thus, it is recommended that the efficiency of the connection and the thickness of the zone of the tubular components which bears the sealing surfaces be preserved and the risks of displacement of the sealing contact points be minimized as far as possible when the connection operates in tension and in compression.
U.S. Pat. No. 7,416,374 describes connections using two threadings in a double helix and in which the thread crests are rounded.
However, that type of solution only improves the fatigue performance. The principle of having two threaded zones in a double start configuration is only used with the aim of accelerating makeup and breakout.
Document U.S. Pat. No. 7,475,917 provides connections in which the angles of the stabbing flanks and the load flanks change along the whole length of the threading.
However, that type of solution, aimed at introducing point changes to the load flank or stabbing flank angles in the threading, can only be used to optimize the distribution of the load concentrations. That type of solution cannot be used to improve the tension/compression performances, to control the pressure of the makeup grease or to reduce the axial displacements which appear under alternating loads.
Document U.S. Pat. No. 6,767,035 is intended to generate a high makeup torque using a dovetail first threading and a positive angle second threading. An alternative described in FIG. 9 proposes the use of two threadings wherein the angles which the stabbing flanks form with the normal to the axis of the connection are equal and highly inclined.
However, those solutions cannot be used to provide good tensile strength and good compressive strength at the same time.